Issue 48

B. Chen et alii, Frattura ed Integrità Strutturale, 48 (2019) 385-399; DOI: 10.3221/IGF-ESIS.48.37 386 after welding. With the continuous improvement of train operation speed, the fatigue failure of the structure is particularly prominent. In the fracture accidents of the frame, most of them are caused by fatigue failure. Therefore, it is more important to accurately evaluate the fatigue strength of the frame than the structural strength. At present, the research on fatigue strength of bogie frame is mainly based on two aspects: dynamics and statics. Dynamics-based research, the dynamic model of multi-rigid-body system of vehicle body is established by analyzing the irregularity of track line, and the load history of vehicle suspension system is obtained. On this basis, the dynamic analysis and stress evaluation of the bogie are carried out, and the fatigue life of the bogie is predicted according to the cumulative damage theory. The difference lies in that different scholars consider different contents in the construction of dynamic model and fatigue life evaluation [1-4]. Statics-based research, researchers mainly use finite element method (FEM) to simulate the load of bogie frame in operational condition and determine the location to be evaluated. Then, the fatigue strength is evaluated based on fatigue limit diagram [5-8]. The difference between the two methods is that the static fatigue strength assessment is based on the fatigue limit diagram and standard loads. The dynamic fatigue strength assessment is based on the measured stress spectrum, and the fatigue life of the frame is predicted by calculating cumulative damage. The results of the former are conservative, while the latter are less accurate because the stress spectrum is difficult to deal with [9]. In view of the above reasons, static method is usually used in the design stage to check the fatigue strength of the frame and the accuracy of calculation is verified by comparison of experimental and simulation results, so as to provide guidance for the design. Currently, there are many reports about fatigue strength analysis of bogie frame at home and abroad. Kim [10] proposed a method of evaluating bogie frame fatigue strength based on GSFLD, which combines multi-body dynamics with structural strength simulation and verified the accuracy of the evaluation through experiments. Different from Kim's assessment method, Wang et al [11] obtains the equivalent stress of key parts of the frame through simulation calculation and line measurement, and evaluates the static strength and fatigue strength according to JIS E 4207 standard. Although the methods of the two scholars are different, they are all based on the nominal stress method for fatigue strength assessment. In order to obtain more accurate calculation results, Wang et al [12] employed hot spot stress method to analyze fatigue strength of bogie welded frame. However, the Wang's method can only be applied to evaluate the welded joints with stress perpendicular to the toe and fatigue crack initiation at the toe. It can't be applied to evaluate the welded joints with fatigue cracks at the weld root and continuous welds subjected to longitudinal loads. Compared with the above methods, Lu et al [13] enhanced the efficiency of structural fatigue strength analysis by comparing the results of finite element analysis of fatigue strength of two different models. The above studies all take the deterministic model as the research object, without considering the uncertainty of design parameters in the process of machining and manufacturing. The results of fatigue strength assessment based on safety factor are conservative, which is not conducive to lightweight design of bogie frame. Therefore, to further reduce the redundancy of fatigue strength analysis of bogie frame, this study modifies traditional approach. In the process of establishing finite element model, the uncertainty of design variables is taken into account. The response surface function of mean stress and stress amplitude are established, and the control points in consideration of uncertainty parameters are calculated. Based on this, the expression of GSFLD function without considering safety factor is derived, and the fatigue strength reliability of control point is calculated with reliability theory. (a) Geometric model of bogie frame (b) Finite element model of bogie frame Figure 1 : Geometric model and finite element model of bogie frame